The work, published Tuesday in the journal Lancet Global Health, was done by scientists from the Liberian Ministry of Health, the Centers for Disease Control and Prevention, and the World Health Organization.

The findings come from Liberia’s Men’s Health Screening Program, set up to test male survivors of Ebola to see if they are still emitting virus in their semen and could potentially infect a sexual partner.

The Ebola virus only has 7 genes and is smaller than a blood cell, but during an infection the deadly disease can shut down multiple organs. Here's how it works.Hyacinth Empinado/STAT

After it became apparent some people in the West African Ebola outbreak were being infected through sex, the WHO advised that male survivors be tested so they would know their status. It advises abstinence or the use of condoms until a man has twice tested negative for the virus.

The researchers reported on results from 429 participants in the program, 38 of whom tested positive at least once for traces of Ebola in their semen.

That ratio — 9 percent — is a bit deceptive. Dr. Mary Choi, one of the lead authors of the article, said the program was only established in July of 2015, six or seven months after the peak of Liberia’s outbreak.

Had the testing begun earlier, Choi said, it would have undoubtedly found more men who were positive. Given how late the work started, finding any was a bit of a surprise, she admitted.

“We just figured we’d be open for a couple of months, and then we would test everybody and everyone would test negative and then we would close,” said Choi, a medical epidemiologist in CDC’s viral special pathogens branch.

“We really didn’t think we would be getting that many people testing positive at that point in the outbreak. But we did. And because of that, we’re still operating today.”

As part of the program, participants worked with trained counselors who advised them on what to do if their results were positive. Choi attributed the program’s success in getting men to come back for repeated testing — especially given the stigma attached to having had Ebola — to the work the counselors did.

Survivors over age 40 were more likely than younger men to have positive semen tests, the researchers reported.

Dr. Daniel Bausch, an Ebola expert working at the WHO, wrote in a commentary that this is the first time age has been identified as a risk factor for long-term Ebola shedding. It might provide a clue, he suggested, as to why this phenomenon happens, suggesting it may be due to age-related changes in the immune system.

Bausch — and the authors themselves — cautioned that just because they found fragments of virus in the semen of these survivors doesn’t mean the men were capable of infecting a sex partner.

The test used in the study detects traces of virus, but cannot indicate if they are part of live viruses — which would be infectious — or are residues of infection that are of no consequence.

One would need to try to grow virus from the semen samples to answer that question. But given how dangerous the Ebola virus is, that work would need to be done in a laboratory with the highest level of biosafety and biosecurity precautions. West Africa doesn’t have such a lab.

Bausch noted a team from the CDC did a similar study, checking the semen of American Ebola survivors. While they found viral fragments out to nine months in one case, they did not manage to grow virus from semen samples after 70 days post-infection.

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Still, until more is known, Bausch said, caution is needed.

“A documented case of sexual transmission in Liberia six months after acute Ebola virus disease, along with accumulating evidence of similar events, remind us that even low levels of virus can result in transmission,” he wrote.

Choi said more research is needed to try to figure out why this happens.

“What we’ve learned from this is that how long this virus stays in each man really does vary,” she said. “And we don’t really understand why that is.”